1. Field of the Invention
The present invention is directed to a device for the frequency conversion of coherent light from a prescribed wavelength range and utilizes at least a crystal consisting of a nonlinear optical material.
2. Prior Art
It is known that the frequency of coherent light sources can be altered with the aid of a polar, dielectric crystal. These are effects of the so-called nonlinear optics. As is known, the polarization P is a function of the electrical field intensity E in accordance with equation: EQU P=.chi.(E) (1)
wherein .chi. is called a "dielectric susceptibility". Equation (1) is provided by the respective materials and thus represents a material equation. Insofar as it deals with a nonlinear relation between the polarization P and the electrical field intensity E, this equation describes, together with the Maxwell equations, processes of nonlinear optics. Distinctly, this equation means that the electrical field intensity in the material, such as the crystal, effects the displacement or shifting of charges, electrons or ions and thereby causes a polarization. In a nonlinear case, this displacement of charges, i.e. the polarization, is not proportional with the electrical field intensity.
The effects of nonlinear optics are described in the following publications: N. Bloembergen, Nonlinear Optics, 1965, New York and an article by Y. R. Shen, "Recent Advavces In Nonlinear Optics", Review of Modern Physics, Vol. 48, No. 1, January 1976, pp. 1-32.
An important process, which uses the nonlinear optics, is the production of harmonic frequencies and the production of sum and differential frequencies. For example, a process of frequency doubling is known. An example is a primary beam strikes or impinges on a crystal having a nonlinear property and, among other things, a secondary beam is produced, which secondary beam has double the frequency relative to the frequency of the primary beam. During the process of producing the sum or the differential frequencies, two primary beams having different frequencies are directed upon a crystal, among other things, a secondary beam, whose frequency corresponds with the sum or the differences of the frequencies of the two primary beams, is formed.
The efficiency and thus the practical usefullness of the above mentioned processes depends primarily upon the phase difference between the primary beams and the secondary beam, for example, upon the phase difference between the producing and the produced light waves. For electro-dynamic reasons, an energy transmission only proceeds in the desired direction when the equation for the phase difference .DELTA..phi. is EQU -.pi..ltoreq..DELTA..phi..ltoreq.+.pi. (2)
The energy transmission is maximum when the equation is: EQU .DELTA..phi.=0 (3)
The phase relationship of a light wave in the crystal is determined by its initial phase, its vacuum wavelength and the respectively determining index of refraction or refractive index n in the crystal. As this refractive index is normally dispersive, for example this refractive index is a function of the light wavelength, the phase difference between two waves involved in an process of optical frequency conversion generally does not disappear. Only optical anisotropy, that is the additional dependency of the refractive index upon the direction and the polarization of the irradiated light wave relative to the dielectric main axes x, y, z of the crystal being utilized, acts in some cases to compensate for the influence of the dispersion and to thus obtain a phase adjustment. The term "phase adjustment" is understood to mean that the phase difference .DELTA..phi. to adjust to the condition set forth in equation (2) and preferably satisfies the ideal case which is set forth in equation (3). How phase adjustment for various crystal types can be produced is described, for example, in the following two publications: J. E. Midwinter and J. Warner, Brit. J. Appl. Phys., 16 (1965), p. 1135 and M. V. Hobden, J. of Appl. Phys., 38 (1967), p. 4365.
It is essential for the understanding of the present invention that the actual refractive index n of a lightwave travelling in a given direction with respect to the principal coordinate system x, y, z of the crystall being used as a nonlinear material is a function of the main refractive indices n.sub.x, n.sub.y, n.sub.z (See J. F. Nye, "Phys. Prop of crystals", Clarendon 1972, p. 236) The main refractive indices are again functions of the wavelength .lambda., and of the temperature T.
Thus the phase difference between two waves travelling in a nonlinear crystal is a function of the direction of the existing wave normals, the wavelengths and the temperature of the crystal.
In the simplest case, that is collinear second harmonic generation in an uniaxial crystal all light rays have the same direction, and moreover, phase matching, if it exists, may be achieved by adjusting only one single critical direction coordinate .theta. exists, so that the phase adjustment is provided then as the primary wave or beam impinges in this angle relative to the optical axes of the crystal. For biaxial crystals in general two critical direction coordinates exist, and the primary beam must impinge at the first critical angle relative to the first axis and must impinge in a second critical angle relative to the second axis.
It must also be taken in consideration that for reasons of energy preservation, the wavelength of the primary beam and of the secondary beam are dependent upon one another, and for a prescribed nonlinear process generally only one independent wavelength variable .lambda..sub.1, for example the wavelength of the primary beam, exist. Insofar as an angle of .theta..sub.0 even exists, by means of which an optical phase adjustment according to the equation (3) can be obtained, this angle is a function f of the characteristics wavelength .lambda..sub.1 and of the crystal temperature T. This can be symbolically written by the following formula: EQU .theta..sub.0 =f(.lambda..sub.1, T) (4).
In the case of the process of collinear frequency doubling, this equation means the following. For a phase adjustment according to equation (3), it is necessary that the primary beam or the secondary beam propagate at a specific angle .theta..sub.0 relative to a specific main axis of the crystal. This angle, the phase watching angle is dependent upon the frequency of either the primary beam or the secondary beam and upon the crystal temperature.
The following difficulties can be concluded from equation (4). If the primary beam is a frequency modulated beam, the phase adjustment angle .theta..sub.0 alters its value with the frequency modulation since .lambda..sub.1, in this case, is not a constant. To correct for this problem, the crystal must be continually rotated in its direction or must be brought to a different temperature. If, for example, the temperature is being changed to correct for the changes in the phase adjustment angle, the time periods involved with changing temperature are typically in the order of 10-100 seconds. If the phase adjustment is adjusted by means of turning or rotating the crystal, the time periods depending on the crystal magnitude are in the order of 0.1-1 second. In addition to the problems which include the technical expense for the synchronous tuning of the phase adjustment, the frequency change or the speed of the optical frequency modulation is severly limited by these forms or types of processes for correcting for changes in the frequency of the primary or secondary beam.